Fractured Fossil to split a map or blueprint will result in the Split tag being added to both items, preventing you from making further copies of either item unless a Mirror of Kalandra is used. A resonator with the Fractured Fossil will reroll the mods on the original item and create a mirrored copy of the original. The fossil should not be used in place of a Mirror of Kalandra to duplicate mirror-tier items, as the original will be randomized.

Five Fractured Fossils can be used to duplicate basetypes. By selling five mirrored copies created by the fossil, to a vendor to receive a non-mirrored, unidentified item. That 5 to 1 recipe will yield a non-influenced copy, while the Item sells for much more to vendors will override vendor recipe. Note that this usage may be obsoleted, as creating one craftable copy need to use 5 Fractured Fossils and add Split modifiers to 5 POE items and potentially lowering the values of the items.